Vertical Cross-Point Memory Arrays

ABSTRACT

A method of manufacturing a memory structure includes forming a plurality of vertically-stacked horizontal line layers, interleaving a plurality of electrically conductive vertical lines with the electrically conductive horizontal lines, and forming a memory film at and between intersections of the electrically conductive vertical lines and the horizontal lines. In one embodiment of the invention, the electrically conductive vertical lines are interleaved with the horizontal lines such that a row of vertical lines is positioned between each horizontally-adjacent pair of horizontal lines in each horizontal line layer. By configuring the electrically conductive vertical lines and electrically conductive horizontal lines so that a row of vertical lines is positioned between each horizontally-adjacent pair of horizontal lines, a unit memory cell footprint of just 2F 2  may be realized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/523,820, filed Aug. 15, 2011, and U.S. Provisional Application No.61/536,515, filed Sep. 20, 2011, both of which are incorporated hereinby reference in their entirety.

BACKGROUND

Non-volatile memory is widely used in portable electronic devices (e.g.,smart phones, personal digital assistants, tablet and notebookcomputers, digital cameras, digital audio players, etc.). Non-volatilememory retains its stored information even in the absence of power. Itis also electrically erasable and reprogrammable, light-weight anddurable, and requires no moving parts. All of these attributes lend wellfor use in portable electronic devices.

To satisfy increasing demand for higher-capacity flash memory whilekeeping manufacturing costs low, flash memory manufacturers haveresorted to process scaling techniques in which the memory cells thatmake up the flash memory—known as “floating gate transistors”—arefabricated with smaller dimensions. By scaling down (i.e., “shrinking”)the individual floating gate transistors, higher capacity flash memoriescan be produced. Over the years, process scaling has proved to beremarkably successful, reducing the minimum feature size of floatinggate transistors from around 1 micron (1,000 nanometers) in the early1990s to around 25 nanometers today. However, the ability to scale downfurther is impeded by diffraction limits of the photolithographyprocesses used in fabricating the floating gate transistors and by shortchannel effects and memory retention problems that arise when floatinggate transistors are scaled down to nanometer dimensions.

To overcome these limitations, various alternative non-volatile memorytechnologies have been proposed. Some of these alternative non-volatilememory technologies have shown great promise. However, challenges tomanufacturing the memory cells in a high-density array remain. Thepresent invention addresses and provides solutions to these challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with respect to accompanying drawings,which are not necessarily to scale and in which like reference numbersare used to indicate identical or functionally similar elements.

FIG. 1 is a perspective drawing depicting a vertical cross-point array(VCPA) formed from two-terminal memory elements, according to anembodiment of the present invention;

FIG. 2 is a sectional view of the VCPA in FIG. 1 through cutting planeA-A;

FIG. 3 is a sectional view of the VCPA in FIGS. 1 and 2 through cuttingplane B-B;

FIG. 4 is a schematic perspective view of a VCPA formed fromtwo-terminal memory elements, according to an embodiment of the presentinvention;

FIGS. 5A-C are schematic drawings depicting how a selected two-terminalmemory elements of the VCPA in FIG. 4 is read, programmed, and erased,respectively;

FIG. 6 is a sectional view of a VCPA formed from conductive metal oxidebased (CMO-based) memory elements, according to one embodiment of thepresent invention;

FIG. 7 is a sectional view of the CMO-based VCPA in FIG. 6 throughcutting plane C-C;

FIGS. 8A and 8B are sectional drawings of a CMO-based memory element ofthe VCPA in FIG. 6 configured in an erased state and a programmed state,respectively;

FIG. 9 depicts how a selected CMO-based memory element of the VCPA inFIG. 6 is programmed during a programming operation;

FIG. 10 depicts how a selected CMO-based memory element of the VCPA inFIG. 6 is erased during an erase operation;

FIG. 11 is a flowchart depicting an exemplary fabrication method thatmay be used to fabricate a VCPA like or similar to the VCPA in FIGS. 6and 7;

FIGS. 12A-H are sectional drawings of a VCPA, like or similar to theVCPA in FIGS. 6 and 7, at various stages in the fabrication methoddepicted in FIG. 11;

FIG. 13 is a sectional view of a completed memory structure thatincludes a VCPA, similar to the VCPA depicted in FIGS. 1-4, and afront-end of the line (FEOL) portion upon which the VCPA is formed;

FIG. 14A depicts a wafer immediately following the performing of an FEOLsemiconductor manufacturing;

FIG. 14B depicts the same wafer as in FIG. 14A immediately following theperforming of a back-end of the line (BEOL) vertical manufacturingprocess on die from the FEOL manufacturing process of FIG. 14A;

FIG. 15 depicts a BEOL portion of a memory structure containing a VCPAsimilar to the VCPA in FIGS. 1-3, highlighting how the center conductors(i.e., local bitlines) of vertical lines of the VCPA are coupled tohorizontal global bitlines, according to one embodiment of the presentinvention;

FIG. 16A is a sectional view of a memory structure containing the BEOLportion in FIG. 15 (through cutting plane G-G of the BEOL portion) andan FEOL portion containing FEOL select transistors and other logiccircuitry used to electrically access the VCPA of the BEOL portion fordata operations;

FIG. 16B is a top plan view of FEOL select transistors positioned on asubstrate and positioned directly under and completely within an areafootprint of a BEOL VCPA fabricated directly over the substrate,according to one embodiment of the present invention;

FIG. 17 is a sectional view of the BEOL memory structure in FIG. 16through cutting plane H-H;

FIG. 18 is a schematic, perspective view of the BEOL portion of thememory structure in FIG. 16;

FIG. 19 is a schematic drawing depicting how use of upper and lower bitline layers in the BEOL portion of the memory structure in FIG. 16 andconnections of the center electrodes of odd and even vertical lines tothe upper and lower bit line layers work to divert a half-select currentI_(HALF) passing through a half-selected memory element away from a bitline used in reading a selected memory element;

FIG. 20 is a sectional drawing of a memory structure that utilizes BEOLvertical field-effect transistors (FETs) formed in BEOL transistorlayers above a VCPA to selectively couple the center conductors (i.e.,local bitlines) of vertical lines in the VCPA to global bitlines in anoverlying global bitline layer, according to an embodiment of thepresent invention;

FIG. 21 is a schematic, perspective view of the BEOL portion of thememory structure in FIG. 20;

FIG. 22 is a sectional drawing of a memory structure that utilizes BEOLvertical field-effect transistors (FETs) formed in BEOL transistorlayers below a VCPA to selectively couple the center conductors (i.e.,local bitlines) of vertical lines in the VCPA to global bitlines in anunderlying global bitline layer, according to an embodiment of thepresent invention;

FIG. 23 is a schematic, perspective view of the BEOL portion of a memorystructure including “upper” vertical select transistors that selectivelycouple the center conductors (i.e. local bitlines) of “odd” verticallines to global bitlines in an upper global bitline layer and “lower”vertical select transistors that selectively couple the centerconductors (i.e., local bitlines) of “even” vertical lines to globalbitlines in a lower global bit line layer, according to one embodimentof the invention;

FIG. 24 is a flowchart depicting a method of forming electricalinterconnect salient steps of an exemplary interconnect formation methodthat may be used to form the electrical interconnect between selecttransistors, center conductors (i.e., local bitlines), global bitlines,and FEOL logic circuitry in the FEOL portion of the VCPA in FIG. 22;

FIG. 25 is a sectional view of a memory structure according to anotherembodiment of the invention that includes vertical BEOL FETs, similar toas the memory structure in FIGS. 16-23, and a VCPA structure having amemory cell footprint of 4F²;

FIG. 26 is sectional view of the memory structure in FIG. 25 throughcutting plane J-J; and

FIG. 27 is a schematic perspective view of a BEOL portion of the memorystructure depicted in FIGS. 25 and 26.

DETAILED DESCRIPTION

Referring first to FIGS. 1-3, there is shown a vertical cross-pointarray (VCPA) 100 manufactured in accordance with an embodiment of thepresent invention. The VCPA 100 comprises a plurality of vertical lines102 extending in the Z-direction, a plurality of horizontal line layerscontaining a plurality of electrically conductive horizontal lines 104extending perpendicularly with respect to the vertical lines 102 (e.g.,in the X-direction in this example), and a plurality of memory cells 106(i.e., memory “cells”) formed in regions where the vertical andhorizontal lines 102 and 104 cross. The vertical lines 102 are arrangedin a grid pattern with a plurality of rows 108 (X-direction) and columns110 (Y-direction) of vertical lines 102 interleaved with the horizontallines 104, such that a row 108 of vertical lines 102 is configuredbetween each consecutive pair of horizontal lines 104. In anotherembodiment, rows 108 of vertical lines 102 are interleaved with thehorizontal lines 104, such that a row 108 of vertical lines 102 ispositioned between every other consecutive pair of horizontal lines 104.

As can be seen more clearly in FIG. 2, which is a sectional view of theVCPA 100 through cutting plane A-A in FIG. 1, and FIG. 3 which issectional view of the VCPA 100 through cutting plane B-B in FIG. 2, eachvertical line 102 comprises an inner (i.e., center) conductor 202surrounded by (or coated in part (i.e., partially coated) in analternative embodiment) by at least one memory film 204, for example,one or more resistive change memory films. The memory film 204 isdisposed between the center conductors 202 of the vertical lines 102 andthe crossing horizontal lines 104, such that the center conductor 202 ofeach vertical line 104 serves as a shared first terminal for memorycells 106 facing left with respect to the centerline 302 of the verticalline 102 (i.e., “left-facing” memory cells) and also as a shared firstterminal for memory cells 106 facing right with respect to thecenterline 302 (i.e., “right-facing” memory cells). Each horizontal line104 that crosses the vertical line 102 serves as second terminal of amemory cell 106 formed between the horizontal line 104 and the centerconductor 202 of the vertical line 102. In other words, a memory cell106 is integrally formed between each horizontal line 104 and the centerconductor 202 of each vertical line 102 that the horizontal line 104crosses. (It should be mentioned that although the memory film 204 inthis exemplary embodiment is formed on the outer surfaces of thevertical lines 102, in other embodiments of the invention, the memoryfilm comprises part of or is formed along the edges of the horizontallines 104, instead.)

FIG. 4 is a schematic perspective view of the VCPA 100 when formed withtwo-terminal resistive change memory cells 406. Each resistive changememory cell 406 is seen to be positioned between a unique centerconductor 202 and horizontal line 104 pair. (As discussed in more detailbelow, the center conductors 202 can be configured to serve as bitlines(or “local bitlines) and the horizontal lines 104 can be configured toserve as wordlines for the VCPA 100.) Further, in each memory layer 103and each left-to-right rank 110 (see FIGS. 1 and 2) of vertical lines102 (i.e., each column), a resistive change memory cell 406 isconfigured between each horizontal line 104 and the center conductor 202of each adjacent vertical line 102. In other words, memory cellconnections are formed on both sides of each horizontal line 104.

It should be mentioned that although the VCPA 100 in FIG. 4 has beendescribed as being formed from resistive change memory cells 406 in thisexemplary embodiment, it could alternatively be formed from other typesof memory cells, whether based on resistive states or some other memorystoring mechanism, whether re-writable or not, and/or whether volatileor non-volatile. For example, the memory cells 106 may alternativelycomprise phase-change (e.g., chalcogenide-based) memory cells,magnetoresistive (i.e., ferromagnetic) memory cells, ferroelectricmemory cells, conductive bridge memory cells, carbon nanotube memorycells, fuse-based memory cells, anti-fuse-based memory cells, or othertype of memory cells.

It should also be mentioned that whereas the VCPA 100 is shown toinclude only five horizontal lines 104 per memory layer 103 and only afour-by-four grid (row×column) of vertical lines 102, this is done toease illustration. In an actual implementation, the VCPA 100 and otherVCPAs described herein would typically have many more horizontal lines104 per memory layer 103 (e.g., hundreds or thousands or more) and manymore vertical lines 102 (e.g., hundreds or thousands or more). Further,whereas the VCPA 100 is depicted as having only four memory layers 103(i.e., a memory “stack” of only four memory layers 103) the memory stackof the VCPA 100 and other VCPAs disclosed herein may be fabricated tohave less than four or more than four memory layers 103, and typicallywould have tens or hundreds of memory layers 103.

Because each resistive change memory cell 406 is associated with aunique center conductor 202 and horizontal line 104 pair, it is possible(although not necessary) to perform data operations (i.e., read, write,program, erase, and restore operations) on any single resistive changememory cell 406 in the VCPA 100. FIGS. 5A-C are drawings illustratinghow data operations may be performed on a “selected” resistive changememory cell 502. As will be understood by those of ordinary skill in theart, the selected resistive change memory cell 502 is selected by adecoder (not shown) which decodes an address identifying the particularvertical and horizontal lines 102 and 104 in the VCPA 100 between whichthe selected resistive change memory cell 502 is disposed. The decoderand other active circuitry used to exercise and control the VCPA 100 arefabricated prior to fabrication of the VCPA 100 in a front-end of theline (FEOL) semiconductor manufacturing process, and the memory cells406 of the VCPA 100 are fabricated in accordance with a back-end of theline (BEOL) process, over the pre-fabricated FEOL structure, as willbecome more apparent in the description that follows. To read theselected resistive change memory cell 502 (FIG. 5A), the horizontal line104 and the center conductor 202 of the vertical line 102 between whichthe selected resistive change memory cell 502 is disposed are biased sothat a read voltage V_(R) is dropped across the selected resistivechange memory cell 502. The read voltage V_(R) has a magnitudesufficient to generate a measurable read current I_(R) through theselected resistive change memory cell 502 but not so high as to alterthe stored memory state of the cell 502. The read current I_(R) has avalue that depends on the resistance of the selected resistive changememory cell 502. Accordingly, when the selected resistive change memorycell 502 is in a high-resistance state, the resulting read current I_(R)is less than when the selected resistive change memory cell 502 is in alow-resistance state. The different resistive states (i.e., low- orhigh-resistance states) are therefore indicative of the stored memorystate (e.g., a logic “0” or logic “1”) of the selected resistive changememory cell 502, i.e., are indicative of whether the selected resistivechange memory cell 502 is in a “programmed” state or an “erased” state.The read current I_(R) or other related signal is directed verticallyalong the center conductor 202 of the selected vertical line 102 to asense amplifier or other measuring circuit, which based on the receivedread current I_(R) or other related signal electrically determines thestored memory state of the selected resistive change memory cell 502,e.g., a logic “0” or a logic “1”.

FIG. 5B depicts how a selected memory cell 502 is programmed. Here, a“programmed” state is defined as corresponding to a high-resistancestate and an “erased” state is been defined as corresponding to alow-resistance state. However, these definitions could be reversed, aswill be appreciated by those of ordinary skill in the art. To programthe selected memory cell 502, a program signal V_(P) is applied acrossthe horizontal line 104 and the center conductor 202 of the verticalline 102 between which the selected resistive change memory cell 502 isdisposed. In one embodiment of the invention, the program signal V_(P)comprises one or more programming pulses having magnitudes greater thanthe read voltage V_(R) and sufficient to alter the resistance of theselected resistive change memory cell 502. The magnitude, durationand/or frequency of the programming pulses are controlled to change theresistance of the selected resistive change memory cell 504 to thedesired high-resistance, programmed state.

How a selected resistive change memory cell 502 is erased (FIG. 5C)depends on whether the selected resistive change memory cell 502 hasunipolar or bipolar switching characteristics. The VCPA 100 may beconfigured to utilize resistive change memory cells having either typeof switching characteristic. For a resistive change memory cell 502having bipolar switching characteristics, an erase signal V_(E)comprising one or more erase pulses opposite in polarity to that of theprogramming pulses of the program signal V_(P) is applied across thecenter conductor 202 and horizontal line 104 so that the erase pulsesare dropped across the selected resistive change memory cell 502. For aresistive change memory cell 502 having unipolar switchingcharacteristics, the polarity of the erase pulses are of the samepolarity as the programming pulses but have magnitudes greater than themagnitude of the read voltage V_(R) but different (i.e., less than orgreater than) the magnitudes of the programming pulses. Some types ofmemory cells have both unipolar and bipolar switching characteristics,in which case erasing may be performed using either a unipolar orbipolar operation. Whether erasing is performed in a unipolar or bipolarfashion, the magnitude, duration and/or frequency of the erase pulsesare controlled so that the resistance of the selected resistive changememory cell 502 is altered to conform to the desired low-resistance,erased state.

It should be noted that the vertical and horizontal lines associatedwith “unselected” memory cells (i.e., those memory cells in the VCPA 100having no horizontal line or vertical line in common with either thehorizontal line or vertical line of a selected memory cell) and thevertical and horizontal lines of “half-selected” and“partially-selected” memory cells (i.e., memory cells that directlyshare either the same vertical line or same horizontal line as theselected memory cell or are indirectly electrically connected to one ofthe vertical or horizontal lines of a selected memory cell) that are notshared with the selected memory cell may be grounded or biased to someother potential (e.g., a floating voltage potential) to prevent orinhibit leakage currents from the half-selected or partially-selectedmemory cells from interfering with the read current I_(R) readoperation. Horizontal and/or vertical lines of un-selected,half-selected and partially-selected memory cells may also be biased toground or some other potential during write operations (i.e., programand erase operations) to prevent or inhibit the resistive states ofun-selected, half-selected and partially-selected memory cells frombeing altered or disturbed during the write operations. Further detailsconcerning methods that may be used or readily adapted to biasunselected, half-selected and partially-selected memory cells duringdata operations are provided in pending U.S. patent application Ser. No.12/657,911, filed on Jan. 29, 2010 and entitled “Local Bit Lines andMethods of Selecting the Same to Access Memory Elements in Cross-PointArrays,” which is hereby incorporated by reference in its entirety forall purposes.

It should also be mentioned that although data operations have beendescribed as being performed on a single selected resistive changememory cell, data operations may also or alternatively be performed on aplurality of memory cells simultaneously. For example, in otherembodiments of the invention, read and program operations may bealternatively performed on a bit, nibble, byte, word, page, block orother higher bit basis and erase operations may be performed on a blockof memory cells or other smaller group of memory cells simultaneously,similar to as in Flash memory. Further, although programming and erasinghas been described as comprising altering the resistance of a selectedresistance change memory cell 406 between two distinct resistivestates—one representing a logic “0” and the other a logic “1”—in otherembodiments of the invention the resistance change memory cells 406 areconfigured as multi-level cells (MLCs). When configured as MLCs,selected resistive change memory cells 406 are configurable to more thantwo resistive states, each resistive state corresponding to one ofseveral stored memory states. For example, each of the resistance changememory cells 406 may be configurable to four different resistive statescorresponding to four distinct storage states: a hard programmed state“00”, a soft programmed state “01”, a hard erased state “11” and a softerase state “10.”

Any type of resistive change memory cell 406 may be used to implementthe memory cells 106 of the VCPA 100 described in FIGS. 1-4 above. FIGS.6 and 7 are sectional drawings similar to the sectional drawings inFIGS. 2 and 3, depicting how conductive metal oxide (CMO)-based memorycells 606 (a type of resistive change memory cell, which may also bereferred to as a tunnel resistive random-access memory element or“tunnel RRAM” element) are used to form a VCPA 600. FIG. 6 is asectional view of the VCPA 600 in the x-y plane and FIG. 7 is sectionalview of the VCPA 600 in through cutting plane C-C in FIG. 6. Eachvertical line 102 comprises a center conductor 602 surrounded by aninner CMO layer 604 and an outer insulating metal oxide (IMO) layer 608(or an inner IMO layer and an outer CMO layer). In an alternativeembodiment, instead of comprising part of the vertical lines 102, one orboth of the CMO and IMO layers 604 and 608 comprise part of or is/areformed along the edges of the horizontal lines 104.

The center conductor 602 of each vertical line 102 in the VCPA 100 iselectrically conductive and serves as a first electrode for memory cells606 associated with that vertical line 102. Each horizontal line 104comprises a conductive line 610 bounded on both sides by an innerdiffusion barrier layer 612, which serves to prevent oxygen diffusionout of the CMO and IMO layers 604 and 608, and an outer edge electrodelayer 614. The horizontal lines 104 are interleaved with the verticallines 102 such that the edge electrode layers 614 of the horizontallines 104 are in contact with the IMO layers 604 of the vertical lines102 at the junctions where the horizontal lines 104 and vertical lines102 cross. This configuration results in CMO-based memory cells 606having CMO and IMO layers 604 and 608 disposed between the centerconductors 602 of the vertical lines 102 and the edge electrodes 614 ofthe crossing horizontal lines 104. Similar to as in the VCPA 100 above,the CMO-based memory cells 606 are formed on both sides of thecenterline 702 of each vertical line 102, as can best be seen in FIG. 7.In other words, “left-facing” CMO-based memory cells are formed to theleft of the centerline 702 and “right-facing” CMO-based memory cells areformed to the right of the centerline 702. Vertical lines 102 are alsopositioned between every consecutive pair of horizontal lines 104, likethe VCPA 100, so that a footprint of 2F² is realized.

The CMO layer 604 of the CMO-based memory cell 606 is an ionic conductorthat can have an amorphous, crystalline, single crystalline, orpolycrystalline structure, or a structure that comprises a combinationof those structures. It may comprise, but is not limited to, amanganite, a perovskite selected from one or more the following:PrCaMnO_(X) (PCMO), LaNiO_(X) (LNO), SrRuO_(X) (SRO), LaSrCrO_(X)(LSCrO), LaCaMnO_(X) (LCMO), LaSrCaMnO_(X) (LSCMO), LaSrMnO_(X) (LSMO),LaSrCoO_(X) (LSCoO), and LaSrFeO_(X) (LSFeO), where x is nominally 3 forperovskites (e.g., x≦3 for perovskites), or a conductive binary oxidecomprised of a binary metal oxide having the form A_(X)O_(Y), where Arepresents a metal and O represents oxygen. The conductive binary oxidematerial may optionally be doped (e.g., with niobium Nb, fluorine F,and/or nitrogen N) to obtain the desired conductive properties for theCMO. The IMO layer 608 is an ionic conductor and an electronicinsulator, and serves as an electrolytic tunnel barrier that ispermeable to oxygen ions during write (i.e., program and erase)operations. It may comprise, but is not limited to, one or more of thefollowing materials: high-k dielectric materials, rare earth oxides,rare earth metal oxides, yttria-stabilized zirconium (YSZ), zirconia(ZrO_(X)), yttrium oxide (YO_(X)), erbium oxide (ErO_(X)), gadoliniumoxide (GdO_(X)), lanthanum aluminum oxide (LaAlO_(X)), and hafnium oxide(HfO_(X)), aluminum oxide (AlO_(X)), silicon oxide (SiO_(X)), ceriaoxide (CeO_(X)), and equivalent materials. Further details concerningthe materials and properties of CMO-based memory cells are described inU.S. patent application Ser. No. 11/095,026, filed Mar. 30, 2005, andpublished as U.S. Pub. No. 2006/0171200, and entitled “Memory UsingMixed Valence Conductive Oxides;” U.S. patent application Ser. No.12/653,836, filed Dec. 18, 2009, and published as U.S. Pub. No.2010/0157658, and entitled “Conductive Metal Oxide Structures InNon-Volatile Re-Writable Memory Devices;” U.S. patent application Ser.No. 11/881,496, filed Jul. 26, 2007, now U.S. Pat. No. 7,897,951, andentitled “Continuous Plane Of Thin-Film Materials for A Two-TerminalCross-Point Memory;” and U.S. Pat. No. 8,003,551, issued on Aug. 23,2011, and entitled “Memory Cell Formation Using Ion Implant IsolatedConductive Metal Oxide;” all of which are hereby incorporated byreference in their entirety and for all purposes.

FIGS. 8A and 8B are cross-sectional drawings depicting the CMO-basedmemory cell 606 in an erased state and a programmed state, respectively.When in an erased state (FIG. 8A), negatively-charged oxygen ions, i.e.,oxygen anions 802 (denoted by the small black-filled circles), aremostly concentrated in the CMO layer 604 and the CMO-based memory cell606 exhibits a low resistance to current (i.e., is in a low-resistancestate). Conversely, when in a programmed state (FIG. 8B), thenegatively-charged oxygen ions 802 are distributed more evenly betweenthe CMO and IMO layers 604 and 608, and the CMO-based memory cell 606exhibits a high resistance to current (i.e., is in a high-resistancestate).

FIGS. 9 and 10 are drawings depicting how the CMO-based memory cell 606is programmed and erased. During a programming operation (FIG. 9), aprogram voltage signal V_(P) comprising one or more programming pulsesis applied across the electrodes 602 and 614 of the selected CMO-basedmemory cell 606. The programming pulse(s) generates an electric field E1in the CMO and IMO layers 604 and 608, forcing a portion of thenegatively-charged oxygen ions 802 in the CMO layer 604 to migrate intothe IMO layer 608 and cause the CMO-based memory cell 606 to conform toa high-resistance, programmed state. (Note that when configured in theVCPA 600 in FIGS. 6-7, the program voltage signal V_(p) is applied viathe horizontal line 104 and the center conductor 602 of the verticalline 102 between which the memory cell 606 is disposed, similar to asdescribed above in reference to FIG. 5B. Erase and read voltage signalsV_(E) and V_(R) are also applied to a selected CMO-based memory cell 606via the memory cell's respective center conductors 606 and horizontallines 104 during erase and read operations (discussed below), similar toas shown in FIGS. 5C and 5A above.)

During the erase operation (FIG. 10), an erase voltage signal V_(E)comprising one or more erase pulses having a polarity opposite that ofthe program pulses of the program voltage signal V_(P) (note that theCMO-based memory cells 606 have bipolar switching characteristics) isapplied across the electrodes 602 and 614 of the selected CMO-basedmemory cell 606. The erase pulses generates an electric field E2,opposite in polarity to that of E1, in the CMO and IMO layers 604 and608, which forces a portion of the negatively-charged oxygen ions 802 tomigrate back out of the IMO layer 608 into the CMO layer 604, resultingin the CMO-based memory cell 606 conforming to a low-resistance, erasedstate.

Once the CMO-based memory cell 606 has been programmed or erased toeither resistive state, it maintains that resistive state, even in theabsence of electrical power. No battery backup or other type of powersource, such as a capacitor or the like, is necessary to retain thestored data. In other words, the CMO-based memory cell 606 isnon-volatile. In addition to being non-volatile, the CMO-based memorycell 606 is re-writable, meaning that it can be programmed and erasedover and over again.

The stored memory state of a selected CMO-based memory cell 606 is readby applying a read voltage V_(R) across its electrodes 602 and 614,similar to as described in reference to FIG. 5A above. The read voltageV_(R) has a magnitude sufficient to generate a measurable read currentI_(R) through the CMO-based memory cell 606 but not so high as to causesubstantial migration (e.g., transport) of mobile oxygen ions betweenthe CMO and IMO layers 604 and 608. The magnitude of the resulting readcurrent I_(R) is dependent upon the resistive state of the CMO-basedmemory cell 606 and a magnitude of the read voltage V_(R). Consequently,when the CMO-based memory cell 606 is in a high-resistance state, theread current I_(R) that results is lower than when the CMO-based memorycell 606 is in a low-resistance state. The read current I_(R) istherefore indicative of the stored memory state (i.e., logic “0” orlogic “1”) of the CMO-based memory cell 606. When the CMO-based memorycell 606 is configured in the VCPA 600, the read current I_(R) or otherrelated signal is directed along the center conductor 602 of thevertical line 102 to a sense amplifier or other measuring circuit, whichelectrically determines the stored memory state of the selected memorycell 606 based on the received signal. It should be noted that theCMO-based memory 406 cells may be alternatively configured for MLCoperation, in which the selected memory cell 606 would be configurableto more than two resistive states, each resistive state corresponding toone of several stored memory states, for example: a hard programmedstate “00”, a soft programmed state “01”, a hard erased state “11” and asoft erase state “10.”

Turning now to FIG. 11, there is shown a flow chart of an exemplaryback-end of the line (BEOL) fabrication method 1100, which may be usedto fabricate a VCPA 1200, similar to the VCPA 600 shown and describedabove in reference to FIGS. 6 and 7. It should be emphasized that thefabrication method 1100 is but one of several ways in which the VCPA 600can be manufactured. Only salient steps of the method 1100 are shown.Further, those of ordinary skill in the art will appreciate andunderstand that the various steps of the method 1100 need notnecessarily be performed in the order shown and that some steps may notbe needed or necessary in all circumstances. Finally, whereas theexemplary method 1100 is described in the context of fabricating a VCPA600 having CMO-based memory cells 606, those of ordinary skill in theart will appreciate and understand that the method 1100 can be readilymodified and adapted to manufacture VCPAs comprised of other types ofmemory cells, including, but not limited to, resistive change memorycells made from other types of materials besides CMO and IMO,chalcogenide-based memory cells, magnetorsesistive (i.e., ferromagneticmemory cells, ferroelectric memory cells, conductive bridge memorycells, memory cells that require a forming process, phase change memorycells, memristive memory cells (e.g., a memristor device), memory cellsincluding multiple layers of memory material where one of those layersis a tunnel barrier layer, memory cells having a non-linear I-Vcharacteristic, memory cells having a non-linear resistancecharacteristic, memory cells having a linear I-V characteristic, andmemory cells having just a single or multiple memory film layers.

At the first stage 1102 of the method 1100 alternating blanket layers ofelectrically conducting and electrically insulating layers 1202 and 1204(e.g., 50-100 nm in thickness each) are formed on a substrate (notshown). As discussed in further detail below, the substrate is asemiconductor substrate or a substrate having a semiconductor layerformed thereon within which some or all of the logic circuitry used tocontrol and perform data operations on memory cells 606 of the VCPA 100has been previously fabricated according to an FEOL semiconductormanufacturing process. The electrically conducting layers 1202 comprisea metal, such as tungsten W, aluminum Al, or tantalum Ta, for example, ametal alloy, or any other suitable electrically conducting material. Theelectrically conducting layers 1202 may be deposited using physicalvapor deposition (PVD) (evaporation, sputtering or ablation of thefilm-forming material), chemical vapor deposition (CVD), in which gases,evaporating liquids, or chemically gasified solids are used as thesource material, atomic layer deposition (ALD), a plating technique suchas, for example, electroless plating, or any other suitable process. Theelectrically insulating layers 1204, which may comprise silicon oxide(SiO_(X)), silicon nitride (SiN_(X)), a combination of SiO_(X) andSiN_(X), a silicate glass (doped or un-doped), or other dielectricmaterial such as a low dielectric constant (i.e., low-k) material, aredeposited using CVD, for example from a TEOS (tetraethylorthosilicate)source, or by vapor phase epitaxy (VPE). The partially completed VCPAstructure following formation of the alternating conducting andinsulating layers 1202 and 1204 is shown in FIG. 12A, which includesboth y-z plane and x-z plane (through cutting plane D-D of the y-zplane) sectional views of the partially completed VCPA structure.

Next, at stage 1104, trenches 1206 extending in the x-direction areformed through the conducting and insulating layers 1202 and 1204, todefine the horizontal lines 104 of the VCPA 1200. The trenches 1206 maybe formed in various ways. In one embodiment of the invention,illustrated in FIG. 12B, trench opening patterns are firstlithographically defined according to a first mask 1208 and thenanisotropically etched using a dry etch process, such as a plasma etch(e.g., a reactive ion etch (RIE)). The partially completed VCPAstructure following stage 1104 is shown in FIG. 12B.

At stage 1106, the trenches 1206 are filled with a second insulatingmaterial 1212 having a high etch selectivity to materials used for theconducting and insulating layers 1202 and 1204, using ALD, for example.Exemplary materials that can be used for the second insulating material1212 include SiO_(X), SiN_(X), or other high-etch-selectivity material.The partially completed VCPA structure following stage 1106 is shown inFIG. 12C.

After the trenches 1206 have been filled with the second insulatingmaterial 1212, at stage 1108 vertical line openings (i.e., “holes”) 1210defining the outer boundaries of the yet-to-be-manufactured verticallines 102 of the VCPA 1200 are patterned and etched in a secondlithography step using a second mask 1214 having features perpendicularto those of the first mask 1208. The second etch is a selective etchthat preferentially etches the second insulating material 1212 accordingto the pattern produced from the second mask 1214 but does not etch (ordoes not substantially etch) other materials like the materials used forthe conducting and insulating layers 1202 and 1204.

The partially completed VCPA structure following stage 1108 is shown inFIG. 12D. It should be noted that the vertical line openings 1210 arenot perfectly vertical. This is due to limits on the ability ofcurrently available etching technologies to form perfectly verticaltrenches. Nevertheless, the vertical line openings 1210 include a highaspect ratio. In some embodiments of the invention, for example, theaspect ratio is between about 20:1 and 80:1, depending on the number ofelectrically conductive and electrically insulating materials 1202 and1204. In the exemplary embodiment shown and described here, the verticalline openings 1210 have a recessive slope 13 (see FIG. 12E) ofapproximately 0.7 degrees, which provides an aspect ratio ofapproximately 80:1 (i.e., 1/sin(0.7):1≅80:1)). It should be emphasized,however, depending on the design and/or processing capabilities, theaspect ratio of the vertical line openings can be less than 20:1 orgreater than 80:1. Future advances in microelectronics processingtechnology may provide for future processes and/or materials thatproduce high aspect ratio openings (e.g., trenches) having vertical orsubstantially vertical sidewall surfaces. Therefore, in someimplementations the openings/trenches described herein may include highaspect ratio structures having vertical or substantially verticalsidewall surfaces.

After the vertical lines openings 1210 have been formed, at stage 1110diffusion barrier layers 612 and edge electrode layers 614 (see FIGS. 6and 7) along the horizontal lines 104 are formed (e.g., using ALD and/ora plating process). To form these layers 612 and 614, recesses 1216 arefirst etched in exposed areas of the electrically conducting layers1202, as shown in FIG. 12E, using a selective etch that preferentiallyremoves only portions of the electrically conducting layers 1202 and notthe electrically insulating layers 1204. Then, the diffusion barrierlayers 612, which may comprise cobalt-tungsten-phosphorus (CoWP) orNickel-phosphorous (NiP), for example, and edge electrode layers 614,which may comprise a noble metal or noble metal alloy such as platinum(Pt) or ruthenium (Ru), for example, are formed in the recesses 1216using selective deposition or plating processes that promotes adhesionto conducting layers 1202 but not to the insulating layers 1204. (Atthis stage in the method 1100, a non-ohmic device (NOD), such as forexample a metal-insulator-metal (MIM) structure or diode(s), may also beformed in each of the recesses 1216. If used, the NODs can serve as aselection device operative to suppress undesirable leakage currents inun-selected or half-selected memory elements). The NOD may include anon-linear I-V characteristic that is separate and apart from any linearor non-linear I-V or non-linear resistance characteristic of the memorycells or memory elements described herein. Completion of stage 1110results in strips of diffusion barrier and edge electrode layers 612 and614 running along the x-z surface edges of the horizontal lines 104, ascan be seen in FIG. 12F. The strips of diffusion barrier and edgeelectrode layers 612 and 614 can also be seen in FIGS. 6 and 7 above.

As alluded to above, although the vertical line openings 1210 have ahigh aspect ratio, realizing very high aspect ratios can be limited bythe capability of available etching technologies, especially in theformation of deep trenches and openings. As illustrated in the FIG. 12E,the vertical line openings 1210 narrow in width from top to bottom asdenoted by widths W1, W2, and W3, such that at the top width W1 isgreater than width W2 midway in the opening which is turn is greaterthan width W3 at the bottom of the opening (i.e., W1>W2>W3). Because theopenings 1210 are not perfectly vertical, the horizontal lines 104,including conductive lines 610, diffusion barrier 612 and edgeelectrodes 614, have sloped profiles. Advances in processingtechnologies may eventually allow the formation of deep trenches 1206and line openings 1210 having much higher or essentially infinite aspectratios, however, resulting in horizontal lines 104 v having verticalfeatures 610 v, 612 v, and 614 v as shown in the right-most drawing inFIG. 12F. For the purpose of this disclosure, therefore, the term“vertical line opening” especially as that term is used in the claims,should be construed as encompassing within its meaning not only trulyvertical line openings but line openings that, though not perfectlyvertical, have a high aspect ratio (i.e., are substantially vertical).

After the diffusion barrier layers 612 and edge electrode layers 614have been formed, at stage 1112 one or more memory film layers, forexample, including but not limited to one or more CMO layer(s) 602 andone or more IMO layer(s) 604 of the CMO-based memory cell 606, areformed on the inner sidewalls of the vertical line openings 1210. Thedeposition technique that is used is preferably a self-limitingconformal deposition technique such as, for example ALD and/or plasmaenhanced ALD (PEALD). ALD allows formation of very thin films that canbe controlled to within about 1 Å. In one embodiment of the invention,ALD is used to deposit one or more thin-film layers of IMO and CMOhaving thickness ranging between about 5-50 Å and about 15-300 Å,respectively. Examples of IMO materials that may be used include, butare not

limited to: YSZ, YOx, ErOx, GdOx, LaAlOx, HfOx, TaOx, ZrOx and Al₂Ox.Examples of CMO materials that may be used include, but are not limitedto PCMO, LNO, SRO, LSCrO, LCMO, LSCMO, LSMO, LSCoO, and LSFeO. After thememory film layer(s) has(have) been deposited, chemical mechanicalpolishing (CMP) may then be employed to remove residual memory filmmaterial that was undesirably deposited on the top surface of thestructure. The partially completed VCPA structure following stage 1112is shown in FIG. 12G.

At stage 1114, a metal, for example a noble metal such as Pt or Ru, isdeposited or plated on the sidewalls of the memory-film-lined verticalline openings 1210, using ALD or a plating process. Then, a conductivebarrier (e.g. Ru) and a bulk metal (e.g., copper (Cu)) are filled in theopenings 1210 to complete formation of the center conductors 602 of thevertical lines 102. The completed VCPA 1200 following completion ofstage 1114 is shown in FIG. 12H.

According to one aspect of the invention, the fabrication method 1100comprises a back-end of the line (BEOL) manufacturing process that isperformed after a front-end of the line (FEOL) semiconductormanufacturing process, in which logic circuitry (e.g., address decoders,data buffers, registers, voltage drivers, memory controller, senseamplifiers, voltage generators, etc.) used to exercise and control theVCPA, is performed. FIG. 13 depicts a cross-section of a memorystructure 1300 that includes a VCPA 100 similar to that depicted abovein FIGS. 1-4 above, in accordance with this embodiment of the invention.Prior to BEOL processing (indicated by the upward-pointing large arrow1330), FEOL processing (indicated by the downward-pointing large arrow1320) is performed to form the various layers of FEOL portion 1301. Inone embodiment of the invention, FEOL processing 1320 comprises acomplementary metal-oxide-semiconductor (CMOS) semiconductormanufacturing process that includes the following salient steps: (1)implanting doped regions of active devices (e.g., transistors diodes)and other circuit elements of logic circuitry 1304 in a semiconductorsubstrate or a semiconductor layer 1306 formed over a substrate 1302;(2) growing gate and gate dielectric layers 1308 over the semiconductorlayer 1306; (3) patterning and etching the gate and gate dielectriclayers 1308 to form gates and gate dielectrics for the active devices;(4) depositing a pre-metal dielectric (PMD) layer 1310 over the gate andgate dielectric layers 1308; (5) depositing, patterning and etchingmetallization and intra-metal dielectric (IMD) layers 1312 over the PMDlayer 1310; (6) forming vias to electrically connect active devices andother circuit elements in the underlying layers; and (7) forming a topinsulating layer 1314 over the metallization and IMD layers 1312.Further details of CMOS fabrication processes that may be used orreadily adapted to form the FEOL portion 1301 of the memory structure1300 may be found in R. Jacob Baker, “CMOS Circuit Design, Layout andSimulation,” Revised Second Edition, IEEE Press, John Wiley & Sons,2008, which is hereby incorporated by reference.

After the FEOL portion 1301 has been fabricated, the VCPA 100 ismanufactured directly on top of the FEOL portion 1301 in BEOL processing1330 BEOL processing 1330 is identical or similar to the VCPAfabrication method 1100 described above in connection FIGS. 11 and12A-H. By growing the VCPA 100 directly on top of the FEOL portion 1301,a unitary integrated circuit comprising monolithically integrated andinseparable FEOL and BEOL portions 1301 and 1303 is formed.

One major benefit of forming the VCPA 100 in a separate BEOL process isthat it affords the ability to form all, substantially all, or asignificant portion of the logic circuitry 1304 beneath the VCPA 100 inthe FEOL portion 1301. This reduces the overall footprint of the memorystructure 1300 (e.g., reduces die size), thereby allowing a large numberof memory structures 1300 to be manufactured across the surface of thesubstrate 1302 (e.g., allows for more die per wafer).

During BEOL processing 1330 (or, alternatively, beforehand during FEOLprocessing 1320), conductive vias 1316 are patterned and etched beneathand/or along the periphery of the VCPA 100 and then filled with aconductive material (e.g., metal) to electrically couple the horizontallines 104 and center conductors 202 of the vertical lines 102 of theVCPA 100 to metal interconnects in the FEOL metallization and IMD layers1312. Additional conductive vias, previously formed through the PMD andgate and gate dielectric layers 1310 and 1308 during FEOL processing1320 (not shown in FIG. 13; see step (6) in the summary of the salientFEOL processing steps describe above), serve to complete the electricalinterconnection of the VCPA 100 to transistors and other devices in thelogic circuitry 1304 of the FEOL portion 1301.

According to one embodiment of the invention illustrated in FIGS. 14Aand 14B, a plurality of memory die 1404 is formed simultaneously acrossthe surface of the substrate (i.e., wafer) 1302. Each memory die 1404includes one or more memory structures like or similar to the memorystructure 1300 in FIG. 13. FIG. 14A shows the wafer 1302 just after FEOLprocessing 1320. At this stage in the process, the wafer 1302 includes aplurality of partially completed memory die 1402, each containing onlythe FEOL portion 1301. The FEOL-processed wafer 1302 is optionallysubjected to FEOL testing 1408 to verify functionality of the logiccircuitry 1304 in the partially completed memory die 1402. Partiallycompleted memory die 1402 that fail FEOL testing 1408 are identified,e.g., by visual marking and/or electronically in a file, database,email, etc., and communicated to the BEOL fabricator and/or fabricationfacility. Partially completed memory die 1402 determined to comply witha specific performance grade (e.g., frequency of operation) may also beidentified, sorted and/or communicated to the BEOL fabricator and/orfabrication facility.

Following FEOL testing 1408, the wafer lot containing wafer 1302 isoptionally transported 1410 to the BEOL fabricator and/or fabricationfacility for subsequent BEOL processing. In some applications both FEOLand BEOL processing 1320 and 1330 are performed by the same fabricatoror are performed at the same fabrication facility, in which casetransport 1410 may not be necessary.

During BEOL processing 1330, the VCPAs 100 are fabricated directly ontop of the upper surface 1405 s of the previously fabricated andpartially completed memory die 1402. It should be emphasized that theVCPAs 100 are not glued, soldered, wafer bonded, or manually attached tothe partially completed memory die 1402. Rather, they are grown directlyon the upper surfaces 1405 s of the partially completed memory die 1402,according to a BEOL fabrication process like or similar to the BEOLfabrication process 1100 shown and described in reference to FIGS. 11and 12A-H above.

FIG. 14B shows the wafer 1302 just after BEOL processing 1330. The wafer1302 includes a plurality of completed memory die 1404 formed across thewafer 1302. The wafer 1302 and completed memory die 1404 are thensubjected to BEOL testing 1412 to verify functionality, measureelectrical characteristics, determine yield, etc. After BEOL testing1412, the memory die 1404 are singulated 1414 (i.e., cut or sawed) intoindividual memory chips 1406. Each singulated memory chip 1406 thatpassed both FEOL and BEOL testing 1408 and 1412 is then optionallypackaged 1416 in an integrated circuit (IC) package, thereby producing apackaged memory chip product 1418. Finally, the packaged memory chipproducts 1418 are subjected to final testing 1420 to verifyfunctionality.

The VCPA 100 and other VCPAs described herein are designed to havegigabit, terabit and even higher memory capacities. To simplify wirerouting and reduce the number of conductive vias 1316 needed toelectrically couple the VCPA 100 to the underlying logic circuitry 1304in the FEOL portion 1301, in one embodiment of the invention thevertical lines 102, specifically, the center conductors 202 of thevertical lines 102, which may be referred to as local bitlines or“LBLs”) are arranged so that they share a reduced number of conductiveglobal bitlines (or “GBLs”). Each LBL is then selected through one ofthe GBLs using decoders configured in the FEOL logic circuitry1304—either directly beneath the VCPA 100 or around its periphery.

FIG. 15 is perspective drawing of the BEOL portion 1503 of a memorystructure 1500, depicting how vertical lines 1505 and 1508 of the VCPA100 are coupled to conductive GBLs 1502, according to one embodiment ofthe invention. FIG. 16A is a sectional view of the memory structure 1500itself through cutting plane G-G in FIG. 15. As shown, the centerconductors 202 (i.e., the LBLs) of the vertical lines 1505 and 1508 arecoupled to GBLs 1502 in either an upper GBL layer 1501 or a lower GBLlayer 1504, by way of conductive vias 1510. More specifically and as isfurther illustrated in FIG. 17, which is sectional view of the memorystructure 1500 through cutting plane H—H in FIG. 16A, and FIG. 18, whichis a schematic perspective view of the BEOL portion 1503 of the memorystructure 1500, the center conductors 202 of the odd vertical lines 1505(odd LBLs) in each left-to-right rank 1506 of vertical lines (i.e., eachcolumn 1506 of vertical lines) are connected to a GBL 1502 in the upperGBL layer 1501 and the center conductors 202 of the even vertical lines1508 (even LBLs) in the same column 1506 of vertical lines are connectedto a GBL 1502 in the lower GBL layer 1504. The vertical line connectionsto the GBLs 1502 in the upper and lower GBL layers 1501 and 1502 couldbe reversed, i.e., so that the center conductors 202 of the odd verticallines 1505 in each column 1506 are connected to GBLs 1502 in the lowerGBL layer 1504 and the center conductors 202 of the even vertical lines1508 are connected to GBLs 1502 in the upper GBL layer 1501. In general,the center conductors 202 of any non-adjacent vertical lines in a givencolumn 1506 of vertical lines 1508 may be connected to a common GBL ineither one of the upper or lower GBL layers 1501 and 1504 with othernon-adjacent vertical lines in the given column connected to a commonGBL 1502 in the other GBL layer. In other words, an even/odd alternationis not required. All that is necessary is that no two adjacent verticallines in a given column 1506 of vertical lines share the same GBL.

As shown in FIG. 16A, GBLs 1502 of both the upper and lower GBL layers1501 and 1504 are electrically coupled to FEOL LBL select transistors1602 (i.e., metal oxide field effect transistors (MOSFETs)) fabricatedin the underlying FEOL portion 1501 of the memory structure 1500, by wayof conductive vias 1510 and metal interconnects 1604. The selecttransistors 1602 operate to electrically couple or decouple the centerconductors 202 of the vertical lines 1505 and 1508 of associated memorycells 106 to decoding or sense circuits in the FEOL logic circuitry1304, depending on which memory cells 106 in the VCPA 100 are selectedduring data operations. Here, dashed lines 1651 demarcate a footprintboundary of the BEOL VCPA in relation to the FEOL substrate layer 1306.As will be described in greater detail below in regards to FIG. 16B, theselect transistors 1602 are positioned beneath the VCPA within theboundaries of the footprint 1651. The GBLs 1502 of the upper GBL layer1501 are formed during BEOL processing, after the VCPA 100 has beenfabricated. The GBLs 1502 of the lower GBL layer 1504 are formed in oneor more of the FEOL metallization layers 1312 during FEOL processing or,subsequently, in one or more other metal layers formed above the topFEOL portion but below the VCPA 100 during BEOL processing.

FIG. 16B is a top plan view of floor planning (e.g., layout orpositioning) of the select transistors 1602 relative to the footprint1651 of the BEOL VCPA. Footprint 1651 is within the die area of thesubstrate 1306 and the select transistors 1602 which are fabricated FEOLwith logic circuitry 1304 are positioned in the FEOL layer entirelywithin the footprint 1651 so that all of the select transistors 1602 arepositioned beneath the VCPA 100 and are electrically coupled (e.g.,1510, 1504, 1501) with their respective vertical lines 1505 and 1508.Placing the select transistors 1602 within the footprint 1651 reducesdie size and allows for reduced feature sizes (e.g., memory cells 106having a 2F² feature size).

FIG. 19 is a schematic drawing illustrating how using the upper andlower GBL layers 1501 and 1504 and alternating local bitline connectionsaid in reading a selected memory cell 1902. A read current I_(R) passingthrough the selected memory cell 1902 is directed vertically in the +zdirection along the center conductor 202 (i.e., the LBL) of itsassociated vertical line 102, horizontally through a GBL 1502 of one anupper GBL layer 1501, and finally vertically in the −z direction througha conductive via 1510 and/or horizontal interconnect (if necessary) thatis/are electrically coupled to a select transistor 1602 in theunderlying FEOL portion 1501. At the same time, an undesired half-selectcell current I_(HALF) passing through the half-selected memory cell 1904on the opposing side of the selected horizontal line 104 is divertedthrough a different GBL 1502 in the lower GBL layer 1504. Diverting thehalf-select cell current I_(HALF) away from the GBL 1502 used in readingthe selected memory cell 1902 allows the stored memory state of theselected memory cell 1902 to be read without being adversely influencedby the half-select cell current I_(HALF), even though the selected andhalf-selected memory cells 1902 and 1904 share the same horizontal line104.

FIG. 19 also depicts how FEOL circuitry 1910 may be implemented toperform data operations such as read and write on selected memory cells.Here, nodes 1920 and 1919 of FETs 1921 and 1923 can be activated toelectrically couple voltage drivers 1931 and 1933 to terminals of theselected memory element 1902 to apply read voltage V_(R) across theselected memory element 1902 thereby generating the read current I_(R)in memory element 1902 and half-select current I_(HALF) in half-selectedmemory element 1904.

In the exemplary memory structure 1500 shown and described in FIGS.15-19 above, the select transistors 1602 used to couple or decouple thecenter conductors 202 of the vertical lines 102 to decoding or sensecircuits in the logic circuitry 1304 are formed in the underlying FEOLportion 1501. Fabricating the select transistors 1602 in the FEOLportion 1501 among all of the other circuit elements of the logiccircuitry 1304 without having to increase the footprint of the VCPA 100can be challenging since the substrate 1306 beneath the VCPA 100 hasonly a limited area. This problem becomes even more challenging thehigher the capacity the VCPA 100 is. Higher capacity VCPAs of the samefootprint have a greater number of memory layers 103 and, consequently,longer vertical lines 102 and a greater number of memory cells 106connected to each vertical line 102. However, the lengths of thevertical lines 102 and the number of memory cells 106 that may beconnected to each vertical line 102 (i.e., the maximum memory cell 106to vertical line 102 ratio) are limited by the amount of tolerablevoltage drop along each vertical line 102 and the amount of leakagecurrent that can be tolerated from half-selected and partially-selectedmemory cells associated with the vertical lines 102 during dataoperations. To avoid exceeding these length andmemory-cell-to-vertical-line ratio limits, the vertical lines 102 can besegmented and connected to additional select transistors 1602 when thelimits are reached. Alternatively, memory capacity can be increased bystacking multiple VCPAs 100 one over the other in the vertical (i.e., +zdirection), such that each VCPA 100 has vertical lines 102 that do notexceed either of these limits. Unfortunately, both approaches toincreasing memory capacity require a greater number of selecttransistors 1602. While the number of excess select transistors may notbe a problem in all circumstances, in circumstances where the availablearea needed to accommodate the additional select transistors is severelyconstrained, the size of the select transistors 1602 must be shrunk,which requires a more aggressive and expensive semiconductormanufacturing process, or the footprint of the memory structure must beincreased. In some cases, neither of these alternatives is particularlydesirable.

FIGS. 20 and 21 are sectional and schematic perspective drawings of amemory structure 2000 (FIG. 20) and the BEOL portion 2003 thereof (FIG.21), according to an embodiment of the invention that avoids the problemof accommodating a large number of select transistors 1602 in theunderlying FEOL portion 2001. According to this embodiment of theinvention, rather than fabricating the select transistors in the FEOLportion 2001, the select devices (e.g., transistor(s), diode(s), NODs,MIMs, etc.) are fabricated in BEOL layers 2020 above the VCPA 100. Forpurposes of explanation, the select devices comprise FETs fabricated ina BEOL transistor layer above the VCPA 100. However, other types of FETsor other non-FET select devices may be alternatively used, as wasalluded to above. The center conductors 202 (i.e., LBLs) of two or morealternating (or non-adjacent) vertical lines 102 (e.g. two or more “odd”vertical lines) in a given left-to-right rank (i.e., column) of verticallines are selectively electrically coupled, via interconnect 2006 in anupper interconnect layer 2018, to a GBL 2004 in a GBL layer 2022 using aselect device 2002 formed in the transistor layers 2020. The centerconductors 202 of two or more different alternating or non-adjacentvertical lines 102 (e.g., two or more “even” vertical lines) in the sameleft-to-right rank of vertical lines (i.e., the same column of verticallines) are selectively coupled to the same GBL 2004 using a differentone of the select device 2002, via interconnect 2008 in a lowerinterconnect layer 2010. The center conductors 202 of the vertical lines102 in the remaining columns of vertical lines 102 are selectivelycoupled to other GBLs 2004 of the GBL layer 2022 in a similar manner, ascan be best seen in the schematic perspective drawing of the BEOLportion 2003 of the memory structure 2000 in FIG. 21. It should bementioned that whereas only four vertical lines per column of verticallines 102 is shown in FIGS. 20 and 21, an actual memory structure wouldhave hundreds, thousands or more of vertical lines per left-to-rightrank, as was explained above. Accordingly, in an actual memory structurethere would typically be many more BEOL select devices 2002 in BEOLtransistor layers 2020. Further, whereas each select device 2002 inFIGS. 20 and 21 is configured to selectively couple the centerconductors 202 of just two non-adjacent vertical lines to a common GBL2004, each select device 2002 could alternatively be configured toselectively couple the center conductors 202 of more than twonon-adjacent vertical lines 102 to a common GBL 2004, depending on thenumber of memory layers 103 and/or how much leakage current fromhalf-selected and partially-selected memory cells can be toleratedduring data operations.

When FETs are used to implement the select devices 2002 of the memorystructure 2000 and other embodiments of the invention, they may compriseplanar or vertical FETs fabricated using FET forming techniques that arewell known to those of ordinary skill in the art. In the embodimentshown in FIGS. 20 and 21, each select device 2002 is formed as avertical FET, such as a gate-all-around FET (i.e., “donut” FET), FinFET,or dual- or multi-gate FET. However, any suitable type of FET may beused. As shown in the magnified view of the vertical FET 2002 in FIG.20, each vertical FET 2002 may comprise a semiconducting channel region2040 of a first conductivity type (e.g., n-type or p-type) bounded ontop and bottom by semiconducting source and drain regions 2042 and 2044of opposite conductivity type; a gate 2046; and a gate dielectric layer2048 formed between the gate 2046 and channel region 2040 that extendsvertically between the source and drain regions 2042 and 2044. The gates2046 of the vertical FETs 2002 are electrically connected to FEOLdecoding circuitry (part of FEOL logic circuitry 1304) in the underlyingFEOL portion 2001, by way of conductive vias (not shown in FIG. 20)formed through the various layers of the BEOL and FEOL portions 2003 and2001. The vertical FETs 2002 function as switches that turn ON and OFFin response to signals generated by the decoding circuitry. It should bereiterated that although FETs are used as the selection devices 2002 inthis and other exemplary embodiments of the invention, other types ofselection devices such as bipolar junction transistors, thin-filmdiodes, metal-insulator-metal devices (MIMs), etc. may be alternativelyused.

In the exemplary memory structure 2000 in FIG. 20, the selecttransistors 2002 are formed in transistor layers 2020 above the VCPA100. In other embodiments of the invention, the select transistors 2002are formed in one or more transistor layers below the VCPA 100, betweenthe upper layer of the FEOL portion 2001 and the bottom of the VCPA 100(as depicted in FIG. 22), below the VCPA 100 in the FEOL portion 2001,both above and below the VCPA 100 (as depicted in FIG. 23), or aroundthe periphery of the VCPA 100 (either in the FEOL portion 2001 or BEOLportion 2003). Similarly, the GBL layers 2022 are formed above the VCPA100, in one or more GBL layers between the lowermost layer of the VCPA100 and the uppermost layer of the FEOL portion 2001, or in the FEOLportion 2001 itself. In the example shown in FIG. 23, “upper” selecttransistors 2302 operate to selectively couple the center conductors 202of two or more alternating or non-adjacent vertical lines 102 (e.g.,“odd” vertical lines) to GBLs 2004 in an upper GBL layer 2301, and“lower” select transistors 2304 operate to selectively couple the centerconductors 202 of two or more different alternating or non-adjacentvertical lines 102 (e.g., “even” vertical lines) to GBLs 2004 in a lowerGBL layer 2305.

FIG. 24 is a flowchart depicting salient steps of an exemplaryinterconnect formation method 2400 that may be used to form theelectrical interconnect between the select transistors 2002, centerconductors 202, GBLs of GBL layer(s) 2222, and FEOL logic circuitry 1304in FEOL portion 2201. The method 2400 is described in reference to theVCPA structure 2200 in FIG. 22. However, it can be readily modified andadapted and used to form the electrical interconnect in the other VCPAstructures disclosed herein. First, in step 2402, a blanketinter-dielectric (ILD) layer 2250 is deposited over the uppermost layerof the FEOL portion 2201. Next, in step 2404, plugs (i.e., vias) 2252are formed through the ILD layer 2250 by: patterning plug openings in afirst lithography step, etching plug recesses into the resulting plugopenings (e.g., using a dielectric ion etch), and depositing a metal(e.g., Cu) barrier, seed and bulk metal into the plug recesses. Theplugs 2404 serve to electrically connect horizontally conducting lines(e.g., the GBLs of GBL layer(s) 2222) and/or devices (e.g., select FETs2002) in given layers of the VCPA structure 2200 to conducting linesand/or devices in other layers of the VCPA structure 2200 (e.g., to FEOLlogic circuitry 1304 in FEOL portion 2201). Following step 2004, in step2406 metal remaining on the surface of the structure is removed, forexample, using a CMP process. Finally, in step 2408 horizontalinterconnects (i.e., “wires”) are formed between metal plugs 2404 byfirst patterning trench openings using a second lithography step,etching trenches according to the defined patterns (e.g., using adielectric ion etch), and depositing a metal (e.g., Cu) barrier, seedand bulk metal into the trenches. Steps 2402-2408 are then repeated orsimilar steps performed, as necessary, to form additional plugs andhorizontal wires, as will be appreciated and understood by those ofordinary skill in the art.

In the VCPAs of the exemplary VCPA structures described above, a row 108of vertical lines 102 is positioned between each consecutive pair ofhorizontal lines 104 and the horizontal lines 104 are configured so thateach horizontal line 104 connects to a vertical line 102 on each of itssides (i.e., edges)—one to the left and another to the right. (See, forexample, FIGS. 1 and 2.) Interleaving the vertical and horizontal lines102 and 104 in that manner yields a memory cell footprint of just 2F²(see FIGS. 3 and 6 above), which in most circumstances is highlydesirable. FIGS. 25-27 depict a memory structure 2500 having a VCPA2502, according to another embodiment of the invention. Like the VCPA100 described above, the VCPA 2502 has vertical lines 102 with centerconductors 202 surrounded by memory film layer(s) 204, horizontal lines104, and memory cells 106, which, like the VCPA 100, may compriseresistive change memory cells (like or similar to the CMO-based memorycell 606 described in FIGS. 6-7), phase-change memory cells,magnetoresistive memory cells, ferroelectric memory cells, conductivebridge memory cells, carbon nanotube based memory cells, etc. However,instead of a row of vertical lines 102 being positioned between eachconsecutive pair of horizontal lines 104, a row 2610 of vertical lines102 is positioned between every other consecutive pair of horizontallines 104 (see FIG. 26). The method of manufacturing the VCPA 2502 issubstantially similar to as described above (see FIG. 11 andaccompanying drawings and description), except that the trenches andholes formed in spaces 2506 are filled with a dielectric prior tocompleting formation of the vertical line openings 1210 and verticallines 102. This results in a larger memory cell footprint 2606 (see FIG.25) of 4F² (i.e., a factor of two larger than the 2F² footprint 206 ofthe VCPA 100) due to the resulting spaces 2506 and forming the verticallines 102 only between every other consecutive pair of horizontal lines104. The GBLs 2004 of GBL layer(s) 2522, like the 2F² VCPA structuresdescribed above, can be fabricated above the VCPA 2502 (as depicted inFIGS. 25-27), in the FEOL portion 2501, or in GBL layers formed bothabove and below the VCPA 2502.

If maximum memory cell density is the primary objective, the increasedcell footprint of 4F² of the memory structure 2500 is not particularlydesirable (compared to the previously described structures, which have acell footprint half the size), especially since the overall memorystructure footprint is multiplicatively increased by the presence ofmultiple spaces 2506. However, in circumstances where it is difficult ornot possible to fabricate all or substantially all of the logiccircuitry 1304 directly beneath the VCPA 2502, as in the other memorystructures described above, the increased footprint may be acceptableand in some cases even desirable. Like the memory structure depicted inFIGS. 20-21, the select devices 2002 (e.g., vertical FETs) used toselectively couple the center conductors 202 of the vertical lines 202to the GBLs 2004 may be formed in BEOL transistor layers 2520 above theVCPA 2502. Alternatively, they may be formed in transistor layersbetween the lowermost memory layer 103 of the VCPA 2502 and theuppermost layer of the FEOL portion 2501 (like the memory structure 2203in FIG. 22), within the FEOL portion 2501, or in transistor layers bothabove and below the VCPA 2502 (like the memory structure 2303 in FIG.23).

Although the present invention has been described in detail withreference to certain exemplary embodiments, various changes in form anddetail are possible. Accordingly, the spirit and scope of the inventionshould not be construed as being limited to specific details or featuresof the exemplary embodiments or to any disclosed embodiment inparticular but instead should be construed in reference to the appendedclaims, including the full scope of equivalents to which such claims areentitled.

What is claimed is:
 1. A method of manufacturing a memory structure,comprising: forming a plurality of vertically-stacked horizontal linelayers, each horizontal line layer including a plurality of electricallyconductive horizontal lines; interleaving a plurality of electricallyconductive vertical lines with the electrically conductive horizontallines such that the electrically conductive vertical lines extendthrough the horizontal line layers; and forming a memory film at andbetween intersections of electrically conductive vertical lines andelectrically conductive horizontal lines to create a memory array oftwo-terminal memory elements.
 2. The method of claim 1, wherein saidelectrically conductive horizontal lines serve as wordlines for thememory array and said electrically conductive vertical lines serve asbitlines for the array.
 3. The method of claim 1, wherein interleavingsaid plurality of electrically conductive vertical lines with theplurality of electrically conductive horizontal lines includes forming arow of electrically conductive vertical lines between everyhorizontally-adjacent pair of electrically conductive horizontal linesin each of said horizontal line layers.
 4. The method of claim 3,wherein said memory array has a memory cell footprint of 2F², where F isa minimum feature size.
 5. The method of claim 1, wherein interleavingsaid plurality of electrically conductive vertical lines with theplurality of electrically conductive horizontal lines includes forming arow of electrically conductive vertical lines between only every otherhorizontally-adjacent pair of electrically conductive horizontal linesin each of said horizontal line layers.
 6. The method of claim 5,wherein said memory array has a memory cell footprint of 4F², where F isa minimum feature size.
 7. The method of claim 1, wherein forming amemory film comprises forming one or more resistive change memory films.8. The method of claim 7, wherein forming one or more resistive changememory films comprises forming one or more insulating metal oxide layersand one or more conductive metal oxide layers.
 9. The method of claim 1,further comprising fabricating: a plurality of global bits lines; aplurality of selection devices operable to selectively electricallycouple electrically conductive vertical lines of said plurality ofelectrically conductive vertical lines, which serves as a plurality oflocal bit lines, to global bit lines of said plurality of global bitslines; and an interconnect structure that electrically connects saidplurality of selection devices to said plurality of global bit lines.10. The method of claim 9, wherein fabricating said plurality of globalbit lines comprises fabricating said plurality of global bit lines belowsaid memory array.
 11. The method of claim 9, wherein fabricating saidplurality of selection devices comprises fabricating said plurality ofselection devices below said memory array.
 12. The method of claim 9,wherein fabricating said plurality of global bit lines comprisesfabricating said plurality of global bit lines above said memory array.13. The method of claim 9, wherein fabricating said plurality ofselection devices comprises fabricating said plurality of selectiondevices above said memory array.
 14. A method of manufacturing avertical cross-point array structure, comprising: depositing a pluralityof alternating layers of electrically insulating and electricallyconducting materials; etching a plurality of trenches through saidplurality of alternating layers of electrically insulating andelectrically conducting materials to define a plurality ofvertically-stacked layers of electrically conductive horizontal lines;filling said trenches with a dielectric material etching a plurality ofholes through the dielectric-filled trenches; etching recesses insections of said electrically conductive horizontal lines that areexposed in said holes; forming an electrically conducting material insaid recesses to form a plurality of first electrodes; forming one ormore memory films on sidewalls of said holes; and depositing anelectrically conducting material in the memory-film-lined holes to forma plurality of second electrodes and an array of vertically-stackedtwo-terminal memory elements between said first electrodes and saidsecond electrodes.
 15. The method of claim 14, wherein forming one ormore memory films on sidewalls of said holes comprises depositing one ormore resistive change memory films.
 16. The method of claim 14, whereinforming one or memory films on sidewalls of said holes comprisesdepositing an insulating metal oxide (IMO) film and a conductive metaloxide (CMO) film.
 17. The method of claim 16, wherein depositing an IMOfilm and a CMO film comprises depositing an IMO film having a thicknessbetween about 5 and 50 Å and a CMO film having a thickness between about15 and 300 Å.
 18. The method of claim 16, wherein depositing said IMOfilm comprises depositing an IMO film according to a self-limitingconformal deposition process.
 19. The method of claim 18, wherein saidself-limiting conformal deposition technique is one that allows said IMOfilm to be deposited at a thickness control of about 1 Å.
 20. The methodof claim 14, wherein etching said plurality of trenches, etching saidplurality of holes, and depositing electrically conducting material inthe memory-film-lined holes are performed so that a row of secondelectrodes is positioned between every horizontally-adjacent pair ofelectrically conductive horizontal lines in each layer of electricallyconductive horizontal lines.
 21. The method of claim 14, wherein etchingsaid plurality of trenches, etching said plurality of holes, anddepositing electrically conducting material in the memory-film-linedholes are performed so that a row of second electrodes is positionedbetween only every other horizontally-adjacent pair of electricallyconductive horizontal lines in each layer of electrically conductivehorizontal lines.
 22. The method of claim 14, wherein etching saidplurality of holes comprises etching a plurality of holes having anaspect ratio of about 20:1 or higher.
 23. The method of claim 22,wherein etching said plurality of holes comprises etching a plurality ofholes having an aspect ratio of between about 20:1 and 80:1.
 24. Amethod of manufacturing a memory structure, comprising: fabricating afront-end of the line (FEOL) portion including logic circuitry foraddressing and performing data operations on memory elements of avertical cross-point array (VCPA), according to an FEOL semiconductormanufacturing process; and fabricating a back-end of the line (BEOL)portion having a VCPA directly over said FEOL portion, according to aBEOL fabrication process that comprises: depositing a plurality ofalternating layers of electrically insulating and electricallyconducting materials on an upper surface of said FEOL portion; etching aplurality of trenches through said plurality of alternating layers ofelectrically insulating and electrically conducting materials to definea plurality of vertically-stacked layers of electrically conductivehorizontal lines; filling said trenches with a dielectric materialetching a plurality of holes through the dielectric-filled trenches;etching recesses in sections of said electrically conductive horizontallines that are exposed in said holes; forming an electrically conductingmaterial in said recesses to form a plurality of first electrodes;forming one or more memory films on sidewalls of said holes; anddepositing an electrically conducting material in the memory-film-linedholes to form a plurality of second electrodes and an array ofvertically-stacked two-terminal memory elements between said firstelectrodes and said second electrodes.
 25. The method of claim 24,wherein forming one or more memory films on sidewalls of said holescomprises depositing one or more resistive change memory films.
 26. Themethod of claim 24, wherein forming one or memory films comprisesdepositing an insulating metal oxide (IMO) film and a conductive metaloxide (CMO) film.
 27. The method of claim 24, wherein forming one ormemory films on sidewalls of said holes comprises depositing one or moreresistive change memory films.
 28. The method of claim 24, whereinforming one or memory films on sidewalls of said holes comprisesdepositing an insulating metal oxide (IMO) film and a conductive metaloxide (CMO) film.
 29. The method of claim 24, wherein etching saidplurality of trenches, etching said plurality of holes, and depositingelectrically conducting material in the memory-film-lined holes areperformed so that a row of second electrodes is positioned between everyhorizontally-adjacent pair of electrically conductive horizontal linesin each layer of electrically conductive horizontal lines.
 30. Themethod of claim 29, wherein said array has a memory cell footprint of2F², where F is a minimum feature size.
 31. The method of claim 24,wherein etching said plurality of trenches, etching said plurality ofholes, and depositing electrically conducting material in thememory-film-lined holes are performed so that a row of second electrodesis positioned between only every other horizontally-adjacent pair ofelectrically conductive horizontal lines in each layer of electricallyconductive horizontal lines.
 32. The method of claim 31, wherein saidarray has a footprint of 4F², where F is a minimum feature size.
 33. Themethod of claim 24, further comprising fabricating: a plurality ofglobal bits lines; a plurality of selection devices operable toselectively electrically couple said plurality of second electrodes,which serves as a plurality of local bit lines, to global bit lines ofsaid plurality of global bits lines; and an interconnect structure thatelectrically connects said plurality of selection devices to saidplurality of global bit lines.
 34. The method of claim 31, whereinfabricating said plurality of global bit lines comprises fabricatingsaid plurality of global bit lines below said plurality ofvertically-stacked two-terminal memory elements.
 35. The method of claim34, wherein fabricating said plurality of global bit lines below saidplurality of vertically-stacked two-terminal memory elements comprisesfabricating said plurality of global bit lines in said FEOL portion aspart of said FEOL semiconductor manufacturing process.
 36. The method ofclaim 34, wherein fabricating said plurality of global bit lines belowsaid plurality of vertically-stacked two-terminal memory elementscomprises fabricating said plurality of global bit lines during saidBEOL fabrication process after said FEOL portion has been fabricated.37. The method of claim 33, wherein fabricating said plurality ofselection devices comprises fabricating said plurality of selectiondevices below said plurality of vertically-stacked two-terminal memoryelements.
 38. The method of claim 37, wherein fabricating said pluralityof selection devices comprises fabricating said plurality of selectiondevices in said FEOL portion as part of said FEOL semiconductormanufacturing process.
 39. The method of claim 37, wherein fabricatingsaid plurality of selection devices comprises fabricating said pluralityof selection devices comprises fabricating said plurality of selectiondevices during said BEOL fabrication process after said FEOL portion hasbeen fabricated.
 40. The method of claim 33, wherein fabricating saidplurality of global bit lines comprises fabricating said plurality ofglobal bit lines above said plurality of vertically-stacked two-terminalmemory elements during said BEOL fabrication process.
 41. The method ofclaim 33, wherein fabricating said plurality of selection devicescomprises fabricating said plurality of selection device above saidplurality of vertically-stacked two-terminal memory elements during saidBEOL fabrication process.